1. Field of the Invention
The present invention relates to a computer memory utilizing method, and more particularly, to a method and apparatus for using a superfluous memory portion of a memory provided on a video graphic adapter (VGA) board as a system memory.
2. Description of the Related Art
A general IBM personal computer (PC) has a memory map and an input/output (I/O) map. The memory map is divided into a system memory area and a video memory area. The video memory is mapped in the system memory. In a VGA, the area that the video memory is mapped in the system memory is limited to A0000h-BFFFFh. The video memory is a frame buffer for storing all the video data displayed on the screen of a monitor, and requires a memory capacity of one megabyte for the VGA in a 1024.times.768.times.256 color mode. Since the amount of memory occupied by the area A0000h-BFFFFh in the system memory is only 128 Kilobytes, the video memory cannot be mapped in the system memory all at one time. Therefore, the video memory is mapped over eight consecutive times in the area A0000h-BFFFFh of the system memory by selection of a map register enable bit and switching. In this method, the video memory can be extended to one megabyte, two megabytes, and four megabytes.
For a VGA in a 1024.times.768.times.16M color mode, however, the video memory of the VGA requires four megabytes. Since the mapped system memory occupies only the area A0000h-BFFFFh, a program for changing the VGA video memory map area and designation of related register values should be continuously operated, thus reducing the operation speed of a computer.
Furthermore, when a 1024.times.768.times.16M color mode is used in a VGA board having a four megabyte-video memory, for example, the four megabytes of memory are exclusively used as the VGA video memory. On the other hand, in a 800.times.600.times.56K color mode, the video memory uses only two megabytes and the remaining two megabytes are superfluous, thereby decreasing memory efficiency.
The reason for such decreased memory efficiency is that the system memory is separated from the VGA video memory, they are controlled by different controllers, and thus they cannot be extended to each other. As described above, in the art, the video memory provided for the VGA is reserved for its own use. Thus, when only a small portion of a given high capacity video memory is used, the superfluous memory portion remains useless, thereby decreasing memory use efficiency.